1. Field of the Invention
The present invention relates to a cleaning method and a polishing apparatus employing such cleaning method, and more particularly to a cleaning method suitable for cleaning substrates that require a high degree of cleanliness, such as semiconductor wafers, glass substrates, or liquid crystal displays, and to a polishing apparatus employing such cleaning method.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recently years, circuit interconnections on semiconductor substrates become finer and the distances between those circuit interconnections have become smaller. One of the processes available for forming such circuit interconnections is photolithography. In the case where circuit interconnections are formed by the photolithography or the like, it requires that surfaces on which patterns images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor substrates flat for photolithography. One customary way of flattening the substrates of the semiconductor substrates is to polish them with a polishing apparatus. As shown in FIG. 8, a conventional polishing apparatus 76 comprises a turntable 72 having a polishing cloth 70 thereon, and a top ring 74 for holding a semiconductor substrate W and pressing the semiconductor substrate W against the turntable 72. In the polishing apparatus, a chemical mechanical polishing (CMP) of the substrate is performed by a combination of chemical polishing with an abrasive liquid and mechanical polishing with abrasive particles contained in the abrasive liquid. An abrasive liquid supply nozzle 78 is provided above the turntable 72 to supply the abrasive liquid Q to the polishing 70. Further, a dressing device 80 is provided to regenerate, i.e., dress, the polishing cloth 70.
FIG. 9 shows a CMP unit which is constructed as an integral unit having the polishing apparatus 76 shown in FIG. 8 and various devices associated with the polishing apparatus 76. The CMP unit has a substantially rectangular shape in plan, and the polishing apparatus 76 is disposed at one side of the CMP unit, and load and unload units 84a, 84b for placing wafer cassettes which accommodate semiconductor substrates to be polished are disposed at the other side of the CMP unit. Transfer robots 86a, 86b are movably provided between the polishing apparatus 76 and the load and unload units 84a, 84b so that the transfer robots 86a, 86b are movable along a transfer line C. Reversing devices 88a, 88b for reversing a semiconductor substrate are disposed at one side of the transfer line C, and cleaning apparatuses 90a, 90b, 90c for cleaning the semiconductor substrate are disposed at the other side of the transfer line C. A pusher 10 is disposed adjacent to the turntable 72 to transfer the semiconductor substrate between the top ring 74 and the pusher 10 by vertical movement thereof.
In the polishing apparatus 76 having the above structure, the semiconductor substrate W is held by the lower surface of the top ring 74 and pressed against the polishing cloth 70 on the turntable 72. The abrasive liquid Q is supplied from the abrasive liquid supply nozzle 78 onto the polishing cloth 70 and retained on the polishing cloth 70. During operation, the top ring 74 exerts a certain pressure on the turntable 72, and the surface of the semiconductor substrate held against the polishing cloth 70 is therefore polished in the presence of the abrasive liquid Q between the surface of the semiconductor substrate W and the polishing cloth 70 by a combination of chemical polishing and mechanical polishing while the top ring and the turntable are rotated. The abrasive liquid Q contains various abrasive particles, and the pH of the abrasive liquid Q is adjusted in accordance with the kind of semiconductor substrates to be polished.
As described above, as semiconductor devices have become more highly integrated, circuit interconnections on semiconductor substrates become finer and the distances between those circuit interconnections have become smaller. Therefore, in the above polishing process, if a particle greater than the distance between interconnections adheres to a semiconductor substrate and thus such particle remains on the product, i.e., semiconductor device, then the particle will short-circuit interconnections on the semiconductor device. Therefore, any undesirable particles on the semiconductor substrate have to be sufficiently smaller than the distance between interconnections on the semiconductor substrate. Such a problem and a requirement hold true for the processing of other substrates, including a glass substrate to be used as a mask, a liquid crystal panel, and so on.
In the above-mentioned CMP process, the semiconductor substrate which has been polished is transferred to the cleaning apparatuses 90a, 90b, and 90c. In the cleaning apparatuses 90a, 90b and 90c, for example, a scrubbing cleaning process in which a cleaning members such as a brush or a sponge, is used to scrub a surface of the semiconductor substrate while supplying a cleaning liquid, such as pure water, and a spinning dry process subsequent to the scrubbing cleaning process are preformed, and the abrasive particles or the ground-off particles attached to the semiconductor substrate during the polishing process are removed from the semiconductor substrate.
When pure water (deionized water) is supplied to the semiconductor substrate which has been polished, the pH of the abrasive liquid remaining on the semiconductor substrate changes greatly. Therefore, in some cases, abrasive particles which have been dispersed in the abrasive liquid having an original pH are aggregated together, and adhere to the surface of the semiconductor substrate. For example, in slurry of colloidal silica which is generally used for polishing a SiO2 layer, silica particles which are abrasive particles are stable in alkali solution having a pH of about 10, and form secondary particles having a diameter of about 0.2, xcexcm due to aggregation of primary silica particles. If this slurry is rapidly diluted with pure water to lower the pH of the slurry to 7 or 8, then the electric potential on the surfaces of silica particles is rapidly changed by so-called pH shock, and the silica particles become unstable to thus aggregate the secondary particles to form larger aggregates. In this specification, the pH shock is defined as a rapid change of a pH. This holds true for the dressing process of the polishing cloth 70. To be more specific, when pure water as a dressing liquid is supplied onto the polishing cloth 70 holding the abrasive liquid Q thereon, the pH of the abrasive liquid is rapidly lowered to cause abrasive particles to aggregate. These aggregates remain on the polishing cloth 70 and cause the semiconductor substrate to form scratches in the polishing process.
It is therefore an object of the present invention to provide a cleaning method which can efficiently perform cleaning of substrates which have been polished without causing abrasive particles contained in an abrasive liquid to be aggregated.
Another object of the present invention is to provide a dressing method which can efficiently perform dressing of a polishing surface on a turntable without causing abrasive particles contained in an abrasive liquid to be aggregated on the polishing surface.
Still another object of the present invention is to provide a polishing apparatus employing such cleaning method or dressing method.
According to a first aspect of the present invention, there is provided a method for polishing a greater than  greater than d then cleaning a substrate. The method comprising polishing a substrate using an abrasive liquid containing abrasive particles, and cleaning a polished surface of the substrate by supplying a cleaning liquid having substantially the same pH as the abrasive liquid or similar pH to the abrasive liquid so that a pH of the abrasive liquid attached to the polished surface of the substrate is not rapidly changed.
In the present invention, when using silica slurry having a pH of about 10 as an abrasive liquid, the cleaning liquid whose pH is in the range of 9 to 11 may be used.
According to the present invention, the pH of the abrasive liquid attached to the substrate in the polishing process is not rapidly changed, and hence cleaning of the substrate is conducted in such a state that the abrasive particles are not aggregated due to pH shock. This cleaning process of the substrate is performed in the case where liquid other than the abrasive liquid is first supplied to the surface of the substrate after the polishing process of the substrate. This cleaning process includes rinsing of the substrate on the turntable or in the vicinity of the turntable by supplying a cleaning liquid to the substrate, and a scrubbing cleaning in which the substrate is scrubbed by a cleaning member while supplying a cleaning liquid to the substrate in a cleaning apparatus.
According to a second aspect of the present invention, there is provided a method for polishing and then cleaning a substrate. The method comprises polishing a substrate using an abrasive liquid containing abrasive particles, and cleaning a polished surface of the substrate by supplying a cleaning liquid whose pH is changed during the cleaning.
According to a third aspect of the present invention, there is provided a method for polishing a substrate and then dressing a polishing surface on a turntable. The method comprises polishing a substrate using a abrasive liquid containing abrasive particles by contacting the substrate with the polishing surface, and dressing the polishing surface by supplying a dressing liquid having substantially the same pH as the abrasive liquid or similar pH to the abrasive liquid so that a pH of the abrasive liquid on the polishing surface is not rapidly changed.
In a preferred aspect, the cleaning liquid or the dressing liquid comprises electrolytic ionic water. Thus, contamination of the substrate caused by a metal ion may be prevented and adjustment of the pH of the abrasive liquid may be made.
According to a fourth aspect of the present invention, there is provided an apparatus for polishing and then cleaning a substrate. The apparatus comprises a polishing apparatus for polishing a substrate using an abrasive liquid containing abrasive particles, and a cleaning apparatus for cleaning a polished surface of the substrate by supplying a cleaning liquid having substantially the same pH as the abrasive liquid or similar pH to the abrasive liquid so that pH of the abrasive liquid attached to the polished surface of the substrate is not rapidly changed.
According to a fifth aspect of the present invention, there is provided an apparatus for polishing and then cleaning a substrate. The apparatus comprises a polishing apparatus for polishing a substrate using an abrasive liquid containing abrasive particles, and a cleaning apparatus for cleaning a polished surface of the substrate by supplying a cleaning liquid whose pH is changed during the cleaning.
According to a sixth aspect of the present invention, there is provided an apparatus for polishing a substrate and then dressing a polishing surface on a turntable. The apparatus comprises a polishing apparatus for polishing a substrate using an abrasive liquid containing abrasive particles by contacting the substrate with the polishing surface, and a dressing apparatus for dressing the polishing surface by supplying a dressing liquid having substantially the same pH as the abrasive liquid or similar pH to the liquid so that a pH of the abrasive liquid on the polishing surface is not rapidly changed.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.